How2Day: The Classification of Herbicides

Herbicides are used to control weeds. Jupiterimages/Photos.com/Getty Images
Herbicides control invasive plants and unwanted weeds. According to the U.S. Fish and Wildlife Service, common characteristics among herbicides are used in their classification. These include such things as active chemical ingredients, how and what part of the plant is affected, the timing of the application and whether it affects only specific plants or anything in its path. Proper handling of herbicides is crucial to ensure there is no risk to human health or the environment.

This refers to how an herbicide affects a plants functioning or metabolism. Within this classification are two subclassifications: contact herbicides and systemic herbicides. Contact herbicides kill only the part of the plant to which it is applied. Systemic herbicides move from the application area, the leaves, roots or stems, to other parts of the plant causing it to die.

Herbicides are grouped into families according to chemical structure. Common families include phenoxy, picolinic acid, sulfonylurea, triazine, uracil and urea herbicides. Phenoxy herbicides can be taken in through the leaves or roots. Picolinic acid herbicides are used to control broadleaf weeds. Sulfonylurea herbicides work slowly to control broadleaf and grass weeds. Triazine, uracil and urea herbicides are absorbed differently, but they all disrupt the plant's photosynthesis.

This refers to where in the plant the herbicide is effective. When applied, it binds to a specific location. This is usually at the cellular level where the herbicide interrupts the growing process. The site of action is not known for many herbicides, and some herbicides have more than one site of action.

This refers to what part of the plant absorbs the herbicide. It can be absorbed through the leaves, stems or roots. Conditions such as the age of the plant, light and wind affect the uptake of herbicides. Herbicides dry out quickly when applied during windy conditions. This results in reduced absorption.

This refers the specific way an herbicide disrupts the development and growing process of the plant. These mechanisms cause different plant symptoms such as stunted growth, leaf discoloration, decay and the killing of plant tissues. Plants can become resistant if the same herbicide with the same mechanism of action is constantly used.

There are selective and nonselective herbicides. Selective herbicides are more effective to some plants than others. For example, if you want to control dandelions without damaging your lawn, use a selective herbicide. Nonselective herbicides affect all weeds, grasses and plants growing in the application area; they interrupt the physical growing processes common to most plants.

Herbicides are classified by how long they stay active in the soil. They can be either persistent or nonpersistent. Persistent herbicides control weeds for a long time, preventing regrowth. A nonpersistent herbicide does not stay in the soil long; they do their job quickly and then become inactive.

Within this classification are post-emergence and pre-emergence herbicides. Post-emergence herbicides work best after the plant has emerged and is visible. This type of herbicide is most effective when sprayed on the plant's leaves. Pre-emergence herbicides are effective on plants before they germinate and start to grow. Pre-emergence herbicides are applied to the soil and watered in thoroughly one to three weeks before the weed seeds germinate.

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